Recently, the miniaturization of various mechanical devices has sparked a new field of technical advancement. Such micro-electromechanical systems (“MEMS”) integrate mechanical elements with electrical control circuits on a substrate, and are typically manufactured using integrated circuit techniques. Because of their small size, MEMS have become increasingly useful in the development of modern, smart products. Common applications include accelerometers, pressure sensors, actuators, and spatial light modulators.
One type of MEMS device involves micromirrors. Micromirror devices utilize an array of thousands or even millions of tiny, tilting reflective surfaces. These micromirrors can be used to reflect light onto a projection surface, typically forming visual images. Used in this way, micromirrors can function as display units, reproducing high quality visual images of the sort needed in up-scale home entertainment devices. They can also be used in optical switching systems and optical communications systems.
Many conventional micromirrors are used within the framework of digital micromirror device (“DMD”) technology. Each of the mirror elements of a DMD may switch between two positions, corresponding to an open or closed light configuration, based on the angle at which the mirror tilts towards the light source. A digital micromirror is in an open position when it is oriented to reflect the light source onto the projection surface. A digital micromirror is in a closed position when it is oriented so that none of the light provided by the light source is projected onto the projection surface. Thus, each digital micromirror can be oriented in either an open or “on” position, or a closed or “off” position, providing a binary or digital response.
By rapidly turning a particular digital micromirror “on” and “off,” the appropriate shade of light can be projected for a particular pixel on the projection surface. And color hues may also be added to a DMD projection system by time multiplexing of the white light source through a color wheel. In practice, digital micromirrors alternate between open and closed positions so fast that the human eye cannot discern the discreet “on” and “off” positions of each digital micromirror. Instead, the human eye extrapolates the discreet binary images projected by each mirror element into a wide variety of pixel shades and hues. In this way, DMDs allow for the accurate reproduction of the whole array of necessary shades and hues by taking advantage of the human eye's averaging of quickly varying brightnesses and colors.
Typically, each micromirror in a DMD is oriented in either the open or closed position using electrostatic forces generated by corresponding electrodes. Commonly, each digital micromirror is located atop a hinge mechanism, and an electrode is located on either side of the hinge. These electrodes are typically formed on a semiconductor substrate beneath the micromirrors. Whenever an appropriate voltage is applied to an electrode, it creates an electrostatic force capable of pivoting the micromirror on its hinge. Only one of the two electrodes will be active at any specific moment in time, corresponding to either the open or closed position.
While many micromirrors are conventionally used in the type of DMD systems described above, micromirrors may also be used within analog micromirror devices. Analog micromirror devices operate using principles akin to those of the DMD, but they differ from DMDs because they do not operate by rapidly switching between two positions corresponding to “on” and “off.” Instead, the appropriate shade of light is transmitted based on the angle of the micromirror in relation to the light source and the projection surface. By altering the angle of incidence within a wide range of available positions, the intensity of light displayed can be adjusted.
In operation, analog micromirrors utilize the same sort of electrostatic attraction as DMDs. Conventionally, each analog micromirror is typically mounted atop a torsion hinge, biased to restore the micromirror back to its neutral position. Electrodes are located under the micromirror. Instead of switching the micromirror positions rapidly between two positions (by pulling the micromirror against one of two electrode contact surfaces using one of two constant electrostatic forces), however, the analog micromirror applies differing levels of electrostatic force in order to bring the forces on the micromirror into equilibrium. For each level of electrostatic force applied by an electrode, the equilibrium point between the electrostatic force and the torsional spring force of the hinge would settle at a distinct angle. So by changing the amount of electrostatic force applied to a conventional analog micromirror, the micromirror's angle of incidence can be altered. By its non-binary nature, the analog micromirror does not require the same rapid back and forth changes in position as the DMD. Instead, it uses a slower but smoother motion to direct the correct shade of light onto the projection surface.
Analog micromirror display units overcome one of the typical problems facing DMDs—the “stiction” problem. DMDs often suffer stiction due to the contact forces present when the electrode pulls the micromirror into position against the electrode contact surface. Since analog micromirrors do not make contact with any surface, they may provide a means for overcoming stiction problems that can affect the contrast available on micromirror video display equipment. And in addition to their uses for video image projection, micromirrors may also be used as optical switching relays within optical wireless communication systems. While DMDs can serve some functions in these sorts of systems, the analog micromirrors are typically used for these purposes since they can provide a greater range of angles of reflection.
Conventional analog micromirrors face their own problems, however. As described above, analog micromirrors generally rely on a balancing of the electrostatic force and the torsional spring force of the hinge in order to orient the micromirror to the appropriate angle. This sort of balancing approach requires that the spring-like resistance applied on the micromirror by the torsional hinge match the electrostatic force applied via the electrode. But while the torsional spring force increases linearly, gaining strength in proportion to its deflection, the electrostatic force increases non-linearly as a function of the square of the distance between the micromirror and the electrode.
The result of the differing nature of the electrostatic force and the linear spring force of the torsion hinge is a phenomenon known as “snap-through.” Once the micromirror has deflected approximately one-third the total separation distance between its neutral position and the electrode, conventional analog micromirrors become unstable as the non-linear electrostatic force rapidly increases beyond the capacity of the linear spring force of the torsion hinge. As the torsion hinge loses the ability to match the electrostatic force, the micromirror can no longer reach an equilibrium position, and instead it suddenly pivots the remaining distance to contact the electrode. Thus, snap-through limits the useful range of motion of a typical analog micromirror device to approximately one-third of the initial separation distance between electrode and micromirror.
Conventional analog micromirrors also have a tendency to degrade over time due to hysteresis. The repeated movement of the micromirror over time influences the responsiveness of the linear torsion spring for a conventional analog micromirror, leading to less precise control of the micromirror. Consequently, overcoming hysteresis would improve the durability of analog micromirror devices.